1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device, and, more particularly, to a semiconductor integrated circuit device fabricated by using a wire bonding technology.
2. Description of the Related Art
Wire bonding is one way of connecting a semiconductor chip to an external substrate or to an external package. This bonding is used to connect an electrode pad provided in a semiconductor chip and an external substrate or the electrode portion of an external package, the connections are made with gold wires about 30 .mu.m in diameter. Today, wire bonding is typical used in assembling semiconductor integrated circuit devices because it is comparatively easy to accomplish and has a high reliability.
Gold wires are bonded to an electrode pad as follows in wire bonding. First, a so-called capillary is positioned above the target electrode pad with a ball about 80 to 100 .mu.m in diameter formed at the end of the gold wire that is to be fed out from the capillary. Then, the capillary descends, bringing the ball into contact with the electrode pad. Ultrasonic vibration is then applied to increase the bonding strength between the gold ball and the electrode pad. The electrode pad should, therefore, be designed with a sufficient mechanical strength and flatness at the bonding surface.
FIG. 1 illustrates the structure of a conventional electrode pad; FIG. 1(a) shows the layout, and FIG. 1(b) being its cross section. Referring to FIG. 1, the electrode pad has a semiconductor substrate 30, a first aluminum film 31, an interlayer insulating film 32, a second aluminum film 33, and a square through hole 32a formed in the interlayer insulating film 32 to enable a connection between the first and second aluminum films 31 and 33. Each side of the through hole 32a is set to be about 90 to 110 .mu.m, larger than the diameter of the gold ball in order, to enhance the strength of the second aluminum film 33 that is to be bonded with the gold ball as well as to ensure the needed flatness of the bonding surface. If each side of the through hole 32a becomes smaller than the diameter of the gold ball, the interlayer insulating film 32 will come under the bonding surface and the bonding pressure applied thereafter might crack the interlayer insulating film 32. The crack would hinder the propagation of ultrasonic vibration and impair the flatness, thus lowering the bonding strength between the gold ball and the electrode pad.
As the sizes involved in the recent semiconductor fabrication has entered into the order of submicrons and as multi-layering is accelerated, some changes on the metallizing process to form an electrode pad are proposed. More specifically, it becomes difficult to stably fill the through holes, formed in the order of submicrons, with the conventional aluminum sputtering alone, so that a through-hole filling step is newly provided prior to the aluminum sputtering step. The through-hole filling step would not be effective unless the through holes have uniform heights and depths. Accomplishing the uniform heights and depths of the through holes requires that the through holes have the same diameter, the aluminum areas directly below the through holes have the same structure, and the layout patterns around the holes are the same.
The diameter of the through hole 32a in the conventional electrode pad in FIG. 1 differs by two figures from that of the through holes used for wiring devices within a chip. Further, the layout pattern around the internal through holes located in the inner area where aluminum wires which are 1 to 2 .mu.m wide run in all directions which clearly differs from that around the through hole 32a in FIG. 1.
If the conventional electrode pad of FIG. 1 is formed in a fabrication process that includes the through-hole filling step, there may be an improper filling of the through hole 32a, raising a problem on the flatness of the electrode pad or the bonding characteristic. If the through hole of the electrode pad is made smaller, as an alternative, it is difficult to obtain an electrode pad with the desired size.